The present invention relates to a combustion control apparatus of a direct-injection diesel engine, and in particular, belongs to a technical field relating to a transient control at a change of a combustion state of engine.
In general, a direct-injection diesel engine injects a fuel into a combustion chamber with high pressure and high temperature at around the top dead center of the compression stroke of cylinder, so that the fuel is burned by its self-ignition. Here, the fuel injected into the combustion chamber proceeds being divided (atomized) into minute liquid drops by a collision with air having high density, and forms substantially a cone-shape fuel spray. The fuel evaporates from surfaces of the fuel drops and forms a fuel mixture by involving air surrounding around mainly a front end and a periphery of the fuel spray. Then, the fuel mixture is self-ignited when it becomes a certain condition with its appropriate concentration and temperature necessary for an ignition, and begins to burn (pre-mixed combustion). Then, it is considered that the portion beginning to burn becomes a core and diffusion combustion is performed involving surrounding fuel vapor and air.
In such a normal combustion of the diesel engine (hereinafter, also referred to as diesel combustion, simply), the initial pre-mixed combustion may be followed by the diffusion combustion that burns most part of fuel. Here, nitrogen oxides (NOx) is produced at a portion, in which an air excessive ratio xcex is nearly 1 in the fuel spray (fuel mixture) having un-homogeneous concentration, due to a rapid generation of heat, and also smoke is produced at a portion, in which a fuel concentration is too high, due to a lack of air. Conventionally, some measures to reduce NOx or smoke are took, such as recirculating part of an exhaust gas into an intake air (Exhaust Gas Recirculation, hereinafter, referred to as EGR, simply) and increasing injection pressure of fuel.
Such recirculating the inert exhaust gas into the intake air system by EGR may suppress production of NOx by decreasing combustion temperature but, on the other hand, promote production of smoke with a large amount of EGR decreasing oxygen in the intake air. Further, increasing injection pressure of fuel may promote minute fuel spray and improve air utilization rate by increasing penetration of the fuel spray, resulting in suppression of smoke, but, on the other hand, it may make a condition where NOx is produced easily. In other words, the conventional combustion of diesel engine provided a trade-off relationship on NOx reduction and smoke reduction, so that it was difficult to reduce both NOx and smoke coincidently.
In contrast, new combustion modes have been recently proposed that provide a combustion sate consisting of pre-mixed combustion mainly by advancing the timing of fuel injection and thereby can reduce NOx and smoke coincidently and greatly. These are generally known as the name of diesel pre-mixed combustion or pre-mixed compression ignition combustion. This is, for example, a new combustion mode, in which a large amount of exhaust gas is recirculated by EGR and a fuel is injected at relatively early timing of the compression stroke of cylinder to mix with air sufficiently, so that the pre-mixed mixture is self-ignited at the end of the compression stroke of cylinder and burns (for example, as shown in Japanese Patent Laid-Open Publication No. 2000-110669).
It is preferable that the rate of recirculated exhaust gas into intake air by EGR (EGR ratio) at such combustion state is set at a much higher level than that at the above-described diesel combustion. That is, a rage amount of exhaust gas having larger thermal capacity than that of air is mixed and thereby density of fuel and oxygen in the pre-mixed mixture is reduced, and as a result, the timing of self-ignition of the pre-mixed mixture may be delayed until near the top dead center of compression of cylinder (TDC) by extending its delay time of ignition. Further, the inert exhaust gas disperses evenly around fuel and oxygen in the pre-mixed mixture and absorbs the heat by combustion, and thereby the production of NOx may be suppressed greatly.
However, because increasing the rate of recirculated exhaust gas in the intake air by EGR means decreasing the amount of air in return, it may be difficult to perform the above-described combustion at an engine operating area where the engine load is relatively high. Thus, conventionally, when the engine operation is at relatively low load, an early fuel injection like the above is performed and EGR ratio is controlled higher than a first predetermined value that is relatively high, resulting in pre-mixed compression ignition combustion. Whereas, when the engine operation is at relatively high load, fuel is injected at around the top dead center by changing fuel injection mode, resulting in diesel combustion. Here, EGR ratio is controlled lower than a second predetermined value that is lower than the first predetermined value in order to suppress an increase of smoke.
In the meantime, in a case where the engine combustion mode is changeable between the pre-mixed compression ignition combustion and the diesel combustion like the above, there exists some problems, such as a transient deterioration of exhaust gas condition at its changing and an occurrence of large noise. That is, when changed from the pre-mixed compression ignition combustion to the diesel combustion, the EGR ratio is changed from a state where it is higher than the first predetermined value to another state where it is lower than the second predetermined value, by reducing the amount of recirculated exhaust gas by EGR. Here, if only fuel injection mode is changed at once to its injection at around TDC for the diesel combustion, the combustion consisting of the diffusion combustion mainly is performed along with an excessive EGR ratio because adjusting controlling the amount of exhaust gas recirculation needs a certain time. As a result, smoke is produced greatly.
On the other hand, in changing from the diesel combustion to the pre-mixed compression ignition combustion, if only fuel injection mode is changed to its early injection when the EGR ratio is not sufficiently high, the fuel may be self-ignited at the too-early timing because adjusting the amount of exhaust gas recirculation needs a certain time as well. As a result, considerably large noise of combustion is produced and an increase of NOx is produced rapidly as well. In addition, a large amount of smoke is produced by combustion of fuel having an insufficient mixture with intake air.
In view of the above-described problems, the present invention has been devised and its object is to suppress a transient deterioration of exhaust gas condition and a production of large noise by applying an effective control to procedures of changing combustion of a diesel engine, in which its combustion is changeable between a first combustion state where its combustion consists of pre-mixed combustion mainly (for example, pre-mixed compression ignition combustion described above) and a second combustion state where its combustion consists of diffusion combustion mainly (for example, conventional diesel combustion).
In order to achieve the above-described object, the present invention provides solution means, in which a farther fuel is injected at a proper timing (latter-injection) after a main combustion begins at around the top dead center of compression of cylinder when a combustion state of diesel engine transfers between the above-described first combustion state and the second combustion state, and thereby smoke produced by the main combustion is burned again by the combustion of this latter-injected fuel.
Specifically, the first aspect of the present invention is premised on a combustion control apparatus of a diesel engine, comprising a fuel injector projecting into a combustion chamber of an engine cylinder, exhaust gas recirculation amount adjusting means for adjusting the amount of recirculated exhaust gas to the combustion chamber, main-injection control means for controlling the fuel injector, so that the fuel injector injects the fuel at least during the intake stroke and/or the compression stroke of cylinder (main-injection) at a first engine operating condition to make a first combustion state where the rate of pre-mixed combustion is greater than that of diffusion combustion, while so that the fuel injector injects the fuel at least at around the top dead center of the compression stroke of cylinder at a second engine operating condition to make a second combustion state where the rate of diffusion combustion is greater than that of pre-mixed combustion, and exhaust gas recirculation control means for controlling said exhaust gas recirculation amount adjusting means, in which an EGR value relating to the amount of recirculated exhaust gas at the first engine operating condition is larger than a first predetermined value, while the EGR value at the second engine operating condition is smaller than a second predetermined value which is smaller than the first predetermined value.
Further, it comprises latter-injection control means for performing a latter-injection when the engine operating condition transfers from one of the first and second engine operating conditions to the other thereof, wherein the fuel is injected by the fuel injector within a predetermined range of the expansion stroke and/or the exhaust stroke of cylinder after the main-injection fuel injected into the combustion chamber by controlling the fuel injector by the main-injection control means begins to burn.
According to the above-described structure, firstly, when the engine operating condition is at the first engine operating condition, the fuel is main-injected at least during the intake stroke and/or the compression stroke of cylinder by controlling the fuel injector by the main-injection control means and the ratio of exhaust gas recirculation becomes larger than a certain value (EGR valuexe2x89xa7first predetermined value) by controlling the exhaust gas recirculation amount adjusting means by the exhaust gas recirculation control means. Thus, the fuel injected at the early timing into the combustion chamber of cylinder disperses considerably and widely in the combustion chamber and mixes sufficiently with air and recirculated exhaust gas, and thereby highly homogeneous mixture can be formed. Accordingly, it makes the first combustion state where the rate of pre-mixed combustion is relatively large with self-ignition at the timing of the end of the compression stroke. In this combustion state, the amount of produced NOx and smoke is very small.
On the other hand, when the engine operating condition is at the second engine operating condition, the fuel is main-injected at least at around the top dead center of the compression stroke of cylinder. Accordingly, it makes the second combustion state where the rate of diffusion combustion is relatively large. Thus, reduction of NOx and smoke can be achieved by the exhaust gas recirculation to the intake air, and enough air supply can be maintained to increase an engine output by the exhaust gas recirculation whose ratio is smaller than a certain value (EGR valuexe2x89xa6second predetermined value).
Further, when the engine operating condition transfers from one of the first and second engine operating conditions to the other thereof, the fuel is latter-injected within a predetermined range of the expansion stroke and/or the exhaust stroke of cylinder after the main-injected fuel begins to burn by controlling the fuel injector by the latter-injection control means. Thus, smoke produced along with the combustion of the main-injected fuel can be burned again during the combustion of the latter-injected fuel, and thereby the concentration of smoke in exhaust gas can be reduced.
Here, in the above-described fuel control apparatus of engine, the timing of the main-injection at the first engine operating condition is advanced from the timing of that at the second engine operating condition (the second aspect of the present invention).
In a combustion control apparatus of engine according to the third aspect of the present invention, the latter-injection control means performs the latter-injection by the fuel injector after a heat release rate by the combustion of the main-injection fuel becomes less than a predetermined value.
By performing the later-injection after the heat release rate by the combustion of the main-injection fuel becomes less than a predetermined value, the combustion of the latter-injected fuel urges the smoke that has been already produced to burn again without producing further smoke by the combustion of the main-injected fuel. Further, because a state is made at that time where smoke is relatively easy to react oxygen without any uneven distribution of smoke in cylinder, burning smoke can be performed effectively. In addition, because the latter-injection is performed at the relatively late timing and thus the temperature of cylinder does not become so high, the combustion of the latter-injected fuel itself may not cause production, growth and condensation of smoke cores. Thus, according to the present invention, the concentration of smoke in exhaust gas can be decreased extremely.
In a combustion control apparatus of engine according to the fourth aspect of the present invention, the latter-injection control means performs the latter-injection by the fuel injector so that the combustion of the latter-injection can begin within a certain range beginning at around a certain point, which is when the heat release rate by the combustion of the main-injection fuel becomes about zero, and ending at another point, which is a predetermined crank angle after the certain point.
Accordingly, the injection timing of the latter-injection is controlled so that the combustion begins based on the completion timing of the combustion of the main-injected fuel, and thereby the function and effect of the above-described third aspect of the present invention can be achieved sufficiently. Here, the range, beginning at around a certain point which is when the heat release rate by the combustion of the main-injection fuel becomes about zero and ending at another point which is a predetermined crank angle after the certain point, means that it is not required that the combustion of the later-injected fuel begins accurately at the timing when the heat generation ends (namely, at the timing of combustion completion). Specifically, it is preferable that the combustion by latter-injection begins within a certain range, for example, a crank angle of xc2x15xc2x0 or xc2x13xc2x0, which is as a center of the point when the heat release rate becomes about zero. Or, the combustion by the latter-injection may begin a little after that.
In a combustion control apparatus of engine according to the fifth aspect of the present invention, the latter-injection control means controls the timing of the latter-injection within a range of about 10xc2x0 CA to about 60xc2x0 CA after the top dead center of the compression stroke of cylinder. The latter-injection within the range can provide the same function and effect as the third aspect of the present invention.
In a combustion control apparatus of engine according to the sixth aspect of the present invention, it farther comprises EGR estimation means for estimating an actual EGR value of the engine, wherein the latter-injection control means is constituted so as to perform the latter-injection by the fuel injector based on the EGR value estimated by the EGR estimation means when the EGR value is within a predetermined range between the first and second predetermined values.
Namely, the fuel latter-injected after the expansion stroke of cylinder as described above forms a combustion state having relatively slow combustion and also a cycle efficiency of engine becomes low. Accordingly, in general the fuel consumption tends to become worse when the latter-injection is performed. According to the present invention, the latter-injection is performed only when it is estimated based on the EGR value that the EGR value is within a predetermined range where a large amount of smoke is produced, at the transition of the engine condition. Thus, it can reduce smoke and suppress such deterioration of fuel consumption as well.
In a combustion control apparatus of engine according to the seventh aspect of the present invention, it further comprises main-injection amount compensation means for reducing the amount of the main-injection fuel so as to offset an increase of torque by the latter-injection when the latter-injection control means performs the latter-injection.
Accordingly, engine torque fluctuation caused by the latter-injection can be suppressed to provide good driving feelings. Specifically, it may be constituted that, for example, the more the amount of the latter-injection is and/or the closer to the top dead center the beginning timing of the latter-injection is, the less the fuel amount of the main-injection fuel is.
In a combustion control apparatus of engine according to the eighth aspect of the present invention, the latter-injection control means controls the amount of the latter-injection fuel, when the engine operating condition transfers from the second engine operating condition to the first engine operating condition and at least after a mode of the main-injection is changed, in such a manner that the greater a difference between a target EGR value and an actual EGR value after the transition of the engine operating condition is, the greater the ratio of the fuel amount of the latter-injection fuel with respect to that of the main-injection is.
Namely, as described above, when the engine operating condition transfers from the second engine operating condition to the first engine operating condition, the mode of the main-injection fuel is changed to its early injection when the EGR ratio is not sufficiently high, and thereby the fuel is self-ignited at the too-early timing. As a result, it may be concerned that considerably large noise of combustion is produced and the exhaust gas condition deteriorates. Here, the greater the difference of the EGR ratio, namely, the difference between a target EGR value and an actual EGR value after the transition of the engine operating condition is, the greater these problems become. In the present invention, however, it is constituted that the greater the difference of the EGR values is, the greater the ratio of fuel amount of the latter-injection fuel with respect to that of the main-injection is. Accordingly, it can be suppressed by reducing the amount of fuel injected at the early timing as much as possible that the fuel is ignited at the too-early timing. Further, even if the early ignition occurs, such noise and deterioration of exhaust gas condition that are caused by it can be suppressed.
In a combustion control apparatus of engine according to the first aspect of the present invention, it is preferable that the first predetermined value of the EGR value is set within a range of about 50 to about 60% of an EGR ratio, while the second predetermined value of the EGR value is set within a range of about 30 to about 40% of the EGR ratio (the ninth aspect of the present invention). Accordingly, good combustion with less NOx and smoke can be achieved at the first and second engine operating condition, respectively.
In a combustion control apparatus of engine according to the first aspect of the present invention, it may be constituted that the latter-injection control means controls the amount of the latter-injection fuel by the fuel injector when the engine operating condition transfers from the second engine operating condition to the first engine operating condition, in such a manner that the greater a target torque of engine is, the more the latter-injection fuel is (the tenth aspect of the present invention). Further, it may be constituted that the latter-injection control means controls the amount of the latter-injection fuel by the fuel injector when the engine operating condition transfers from the second engine operating condition to the first engine operating condition, in such a manner that the greater a engine speed is, the more the latter-injection fuel is (the eleventh aspect of the present invention). Further, it may be constituted that the latter-injection control means controls the timing of the latter-injection by the fuel injector according to a target torque of engine and an engine speed when the engine operating condition transfers from the second engine operating condition to the first engine operating condition, in such a manner that the greater the target torque of engine is and/or the lower the engine speed is, the more advanced the timing is (the twelfth aspect of the present invention).
In a combustion control apparatus of engine according to the third aspect of the present invention, it may further comprise a map in which the timing of the latter-injection is preset correlating the engine operating condition, based on experiments and the like, in consideration of an ignition-delay time of the latter-ignition fuel so that the latter-injection fuel can begin to burn at the point of the combustion completion of the main-injection fuel. The latter-injection control means may control the timing of the latter-injection in reference to this map according to the engine operating condition (the thirteenth aspect of the present invention).
In a combustion control apparatus of engine according to the sixth aspect of the present invention, the latter-injection control means may be constituted so that the latter-injection control means can begin the latter-injection by the fuel injector when the EGR value estimated by the EGR estimation means becomes a first middle value between the first and second predetermined values when the engine operating condition transfers from the first engine operating condition to the second engine operating condition, while the latter-injection control means can begin the latter-injection when the EGR value estimated by the EGR estimation means becomes a second middle value different from the first middle value when the engine operating condition transfers from the second engine operating condition to the first engine operating condition (the fourteenth aspect of the present invention). Thus, stability of its control can be obtained.
In a combustion control apparatus of engine according to the seventh aspect of the present invention, it may be constituted that, for example, the more the amount of latter-injection fuel is, the less the amount of main-injection fuel compensated by the main-injection amount compensation means is (the fifteenth aspect of the present invention). Further, it may be constituted that the more the timing of the latter-injection is advanced, the less the amount of main-injection fuel compensated by the main-injection amount compensation means is (the sixteenth aspect of the present invention).
The seventeenth aspect of the present invention is premised on a combustion control apparatus of a diesel engine, comprising a fuel injector projecting into a combustion chamber of an engine cylinder, an exhaust gas recirculation amount adjusting valve adjusting the amount of recirculated exhaust gas to the combustion chamber, a sensor for sensing an engine speed, a sensor for sensing the amount of intake air of engine, a sensor for sensing the amount of accelerator operation, and a control unit for controlling the fuel injector and said exhaust gas recirculation amount adjusting valve at least according to signals from the sensors.
Further, the control unit comprises operating area determination section that determines at least according to the engine speed and the accelerator operation amount whether the engine operation is at a first engine operating area where the engine operation is relatively at low engine load and low engine speed or a second engine operating area where the engine operation is relatively at high engine load and/or high engine speed, main-injection control section that controls said fuel injector, so that the fuel injector injects the fuel during the intake stroke and/or the compression stroke of cylinder at the first engine operating area, while so that the fuel injector injects the fuel at the timing which is delayed from the timing of the first operating area at the second engine operating area, exhaust gas recirculation control section that controls the exhaust gas recirculation amount adjusting valve, in which an EGR value relating to the amount of recirculated exhaust gas at the first engine operating area is larger than a first predetermined value, while the EGR value at the second engine operating area is smaller than a second predetermined value which is smaller than the first predetermined value, latter-injection control section that performs a latter-injection when the engine operation transfers from one of the first and second engine operating areas to the other thereof, wherein the latter-injection fuel is injected by the fuel injector after the main-injection fuel injected into the combustion chamber by controlling the fuel injector by the main-injection control section begins to burn and when a heat release rate by the combustion of the main-injection fuel becomes less than a predetermined value, and main-injection amount compensation section that reduces the amount of the main-injection fuel so as to offset an increase of engine torque by the latter-injection when the latter-injection control section performs the latter-injection.
According to the above-described structure, the same function and effect as the first aspect of the present invention can be achieved.
As described above, according to the first aspect of the present invention, in which a combustion state is changeable between the first combustion state having relatively the greater rate of pre-mixed combustion and the second combustion state having relatively the greater rate of diffusion combustion according to the engine operating condition, when the combustion state transfers from one of the first and second combustion states to the other thereof, further fuel is latter-injected after the main-injection begins at around the top dead center of compression of cylinder, so that the latter-injected fuel can burns smoke in exhaust gas again and thereby an transient increase of smoke can be suppressed.
According to the third aspect of the present invention, the latter-injection is performed after the heat release rate by the combustion of the main-injection fuel becomes less than the predetermined value. Thus, the smoke that has been already produced can be burned again efficiently without producing further smoke by the combustion of the main-injected fuel, and further producing smoke by the combustion of the latter-injected fuel can be avoided. Accordingly, the concentration of smoke in exhaust gas can be decreased extremely.
According to the sixth aspect of the present invention, the latter-injection is performed only when it is necessary based on a state of actual exhaust gas recirculation at the transition of the engine operating condition. Thus, deterioration of fuel consumption can be suppressed with reducing smoke.
According to the seventh aspect of the present invention, the amount of the main-injection fuel is reduced so as to offset the increase of torque by the latter-injection, and thus good driving feelings can be obtained.
According to the eighth aspect of the present invention, when the engine operating condition transfers from the second engine operating condition to the first engine operating condition, the greater the difference of the EGR values is, the greater the ratio of fuel amount of the latter-injection fuel with respect to that of the main-injection is. Thus, it can be suppressed that the main-injected fuel is ignited at the too-early timing, and also noise and deterioration of exhaust gas condition and the like that are caused by it can be suppressed.
According to the fourteenth aspect of the present invention, the timing of beginning the latter-injection is differed between when the engine operating condition transfers from the first engine operating condition to the second engine operating condition and when the engine operating condition transfers from the second engine operating condition to the first engine operating condition. Thus, stability of its control can be obtained.